By the end of the craze sure, but to kick start it and get the opening few million that caused banwagonners to hop on they had to appeal to as wide a range as possible of people and Gen 1 was chosen for that reason.
Like This is straight from their mouths and nearly all their actions around it support their stance from their ad's to their interviews. The Google game that inspired and opened the door to Pokemon had pokemon from multiple gens and
THEY TOOK THOSE POKEMON OUT in favor of Gen 1 only. Like I've said before, I don't think this played the largest role in pokemon go getting big, but it definitely played had an impact.
Anyway here's some fun info
NS demographic break down.
And here's pokemon go near the height of its craze.
Age range is most interesting part of these to me.